1. Field of the Invention
The present invention relates to a drying section for drying a moving web in a web-handling apparatus, the drying section formed preferably as a part of a paper-making machine.
2. Description of the Prior Art
A drying section is described in German Patent DE 3,623,971 as having a number of drying cylinders disposed to form a two-felt drying group comprising an upper and a lower row of cylinders. In each row of cylinders, at least two adjacent cylinders form a sub-group. A deflection suction roll, which guides a felt, wire or like component (hereinafter referred to generally as a "felt"), together with a web to be dried, from the first to the second cylinder of the sub-group, is provided between the two cylinders. After passing through a sub-group (for example in the lower row of cylinders), the web runs over a so-called "web path" to a sub-group of the upper row of cylinders. These two sub-groups overlap one another so that the second cylinder of the lower sub-group lies beneath the first cylinder of the upper sub-group. After passing through the upper sub-group, the web again transfers downwardly to a further sub-group of the lower group of cylinders. It is also known from the above-noted German Patent to provide guide rolls for guiding the respective felts to one of the sub-groups and from there to the following sub-group (in the same row of cylinders). These guide rolls are disposed along the "web path" so that the web to be dried is guided a short distance by the felt, then runs freely for a short distance and then is conveyed for a short distance by the felt of the other row of cylinders to the following cylinder. The arrangement is also such that in the region of one deflection suction roll of one row of cylinders, there is located a predetermined section of the felt path of the other row of cylinders which is free from the web and touches two felt rolls. An air-blast box is also provided for blowing dry air onto the web to be dried at a point where the web runs together with the respective felt over the deflection suction roll. The air-blast box is disposed between the deflection suction roll and the above-mentioned predetermined section of the felt path.
On the one hand, the drying efficiency of the known drying section can be increased with the assistance of the air-blast boxes mentioned above. On the other hand, a problem occurs in the so-called pockets, which are very narrow, if the web to be dried tears. If the web to be dried tears, pieces of the torn-off web (the so-called "broke") remain hanging on the air-blast boxes. It is then normally necessary to stop the drying section to remove the broke. A further complication is that the air-blast boxes cannot be swivelled away (or can only be swivelled a short distance) from the deflection suction rollers because of the restricted spatial conditions.
An advantage of the drying section arrangement discussed above lies in that the number of the so-called "web paths" (between the upper and the lower row of cylinders) is roughly one half less than in the two-felt drying group normally used in a papermaking machines, in which, after each single cylinder, the web transfers from one row of cylinders to the next. The problem of web tears occurs therefore substantially less frequently than in conventional drying sections, but is in no way completely solved.